A) Field of the Invention
The present invention relates to a display device and its driving method, and more particularly to a display device for displaying an image by rotating rotatable particles disposed in a spatial area by applying an electric field and to its driving method, each rotatable particle having at least two surface regions having different colors and electrical characteristics.
B) Description of the Related Art
Sheet-like display media and devices have been proposed which are called electronic paper, paper-like display, digital paper or the like and which display an image by changing an optical absorption or reflection factor by applying an electric field. JP-A-1-86116 discloses a display device having microcapsules each accommodating colored solvent dispersed with electrophoretic particles. JP-A-8-234686 discloses a display device having microcapsules each accommodating a rotatable member of two semi spheres having different colors and electrical characteristics. A display device is also known having a liquid crystal-polymer composite film which contains dichroic pigment and semantic liquid crystal.
These display devices have a memory performance capable of holding image information without a power supply. Further, since a display layer can be formed on a PET film with electrodes, these display devices are expected to form a sheet-type display device which is thin, light and flexible.
A display medium described in the specifications of U.S. Pat. Nos. 4,126,854 and 4,143,103 has a contrast superior to other media. This display medium has rotatable particles each having two semi sphere regions having different colors and charging characteristics.
The operation principle of the display medium described in U.S. Pat. No. 4,126,854 will be described with reference to FIGS. 6A, 6B and 6C.
A plurality of voids 101 are formed in an optically transparent base material 100. A rotatable particle 103 is accommodated in each void 101 and translucent liquid 102 is filled therein. Two semi sphere regions having different colors and charging characteristics, e.g., white and black regions, are defined on the surface of each rotatable particle 103. The surface of the rotatable particle 103 is electrically charged by friction with the translucent liquid 102. The surface charge density of the white region is different from that of the black region.
Therefore, as an electric field is applied to the void 101, the rotatable particle 103 is transferred in electrophoresis and rotated. For example, FIG. 6A illustrates the state that the rotatable particle 103 sinks to the bottom of the void 101, with the white region being directed downward. As an electric field is applied, as shown in FIG. 6B the rotatable particle 103 floats and rises up while being rotated. Lastly, as shown in FIG. 6C the rotatable particle 103 rises to the top of the void 101, with the white region being directed upward.
When the display device in the state shown in FIG. 6A is looked at from the upper position in FIG. 6A, a black color can be visually recognized, and in the state shown in FIG. 6C, a white color can be visually recognized. In this manner, by rotatively moving the rotatable particle 103, the color can be switched between black and white.
Next, a conventional manufacture method for the rotatable particle 103 will be described. The specification of U.S. Pat. No. 5,262,908 discloses a method of coupling two types of melted wax particles having different colors, rounding them by a surface tension, and then solidifying them. The material of the rotatable particle is carnauba wax which contains carbon black, carnauba wax which contains titanium oxide, or the like.
A method of vapor-depositing or coating metal, carbon black, antimony or the like on the surface of a translucent particle is disclosed in JP-A-11-85067 and JP-A-9-246739. The material of the translucent particle is glass or resin.
JP-A-11-85069 discloses a method of manufacturing a rotatable particle by corona-charging the surface of a zinc oxide sphere, exposing the semi sphere region, and reverse-developing it with black toner. JP-A-11-161206 discloses a method of manufacturing a rotatable particle by exposing a partial surface of an opaque photosensitive particle which contains silver salt halide, coloring the exposed area by dipping it in developing liquid and fixing it.
A plurality of rotatable particles are rotatably held in a sheet and electrodes are formed on both sides of the sheet to form a display device. By applying a voltage across the electrodes, an electric field is applied to the inside of the sheet to rotatively move each rotatable particle.
As a d.c. voltage pulse is applied across the electrodes of a display device, there arises a variation in a rotation operation of each rotatable particle so that a clear image cannot be displayed.